1. Field of the Invention
This invention relates to a dental root canal therapeutic instrument for forming a root canal in dental treatments, such as, so-called, a reamer and a file.
2. Description of Related Art
Root canals in teeth are very fine and have delicately curving shapes, which are greatly different among persons. Reamers and files have been used as therapeutic instruments for grinding and forming such root canals. The reamers and files are grinding instruments having cutting edges extending spirally, and a contour line connecting apexes of the cutting edges is in a tapered shape. The reamers are mainly used for grinding the root canals by rotary manipulation, and the files are mainly used for grinding root canals by pushing and pulling manipulation. There are, among the files, a K-file capable of grinding with rotary motion with a relatively slight torsion angle, an H-file exclusively for pushing and pulling grinding with the greatest torsion angle, and so on.
Now, cross-sectional shapes of work portions of a K-file and an H-file currently provided are described. FIG. 10 is a transversal cross section of a work portion of a K-file; FIG. 11 is a vertical cross section of the work portion. The circles shown in FIG. 10 are circles in inwardly contacting with cutting edges located at an arbitrary position of the file. Numeral 51 in FIG. 10 is a file having a square transversal cross section, whose vertical cross section is shown in FIG. 11. The file 51 among commercially available files has a higher cross-sectional secondary moment. The file 51, therefore, indicates high resistance against bending and torsion but has a small rake xcex8 of the cutting edge 51a, so that the file has inferior grinding ability and removing ability of shavings, and also, has lower tracing ability to root canals. The rake xcex8 means that the perpendicular direction to the moving direction of the cutting edges is zero degree, and it is a minus angle if the rake plane, or the cutting edge plane in the moving direction, is inclined toward the moving direction and a plus angle if toward the opposite direction. The file 51a, accordingly has the rake of a minus angle, and the angle xcex8 is represented as small because the angle is minus with a large absolute value.
Numeral 52 shown in FIG. 10 is a file having triangle transversal cross section. The file has a smaller cross-sectional secondary moment in comparison with the above file 51 and a good tracing ability to the root canals. The file can form a large space between the circle and the surface where the file has a large rake of the cutting edge, so that the file can have good grinding ability and removing ability of shavings.
As a general demand for files, raised are: ability to trace flexibly to any bending shape of root canals, which have large personal differences, good grinding ability, ability to easily remove shavings in accordance with rotary manipulation or pushing and pulling manipulation, good resistance against bending in proper response to the root canal shape or instrument size, high breakdown angle property against torsion, ability not to be locked upon engagement especially during rotation, and so on.
Such a root canal therapeutic instrument can be manufactured by forming a liner material having a diameter corresponding to the targeted size and by twisting the material in adapting a method disclosed in, e.g., Japanese Patent Publication Showa No. 62-22,733 after the outer periphery of the material is formed into a shape with the targeted transversal cross section by slantly grinding in a longitudinal direction by a method disclosed in, e.g., Japanese Patent Publication Showa No. 58-52,782.
The root canal therapeutic instruments, particularly files, grinds the root canal wall according to pushing and pulling manipulation. As shown in FIG. 11, however, the cutting edges in contacting with the root canal wall in the depth direction have approximately the same angles on the pulling side and the pushing side (in a direction along the depth of the root canal, or in a direction along the vertical direction of the file). The file has nearly the same grinding ability in pushing manipulation and in pulling manipulation, so that there raises a problem that the shavings generated along the pushing manipulation may reach a root apex opening and enter into the patient""s body. Where the shavings reach the root apex opening and where bacteria thus enter into the human body from the root apex, the bacteria may cause inflammation on the patient, may be accompanied with gross pains, or may cause swellings, and when such a swelling is so severe, the patient may problematically suffer from great pains as his face shape is deformed.
It is an object of the invention to provide a dental root canal therapeutic instrument being flexible, having good grinding ability, and showing grinding ability only during pulling manipulation while not grinding during pushing manipulation, and to provide a manufacturing method for the dental root canal therapeutic instrument.
To solve the above problems, a dental root canal therapeutic instrument of the invention has a shaft portion and a work portion made of spiral projections in continuation with the shaft portion; the cross section of the work portion is formed of a set of long sides and a set of short sides and is composed of a parallelogram constituted of a set of acute angle edges and a set of obtuse angle edges; the acute angle edges among the acute angle edges and the obtuse angle edges, which form the projections, are placed on a side of the shaft on the side face of the work portion; the tip of the acute angle edge is located at a further position with respect to the center axis of the work portion than the position of the tip of the obtuse angle edge.
Since the transversal cross section of the work portion is the parallelogram, the tip of the acute angle edge is located at a further position with respect to the center axis of the work portion than the position of the tip of the obtuse angle edge. Therefore, when the acute angle edge is made in contact with a targeted circle, the obtuse angle edge is placed without contacting the circle as the apex is located inside the circle. Where the acute angle edge, between the pair of the edges placed in the longitudinal direction at the spirally twisted work portion and placed adjacent to each other, is located on a shaft side, the obtuse angle edge disposed on the tip side of the work portion has an apex lower than the apex of the acute angle edge. Accordingly, even where the acute angle edge contacts with the root canal wall, the obtuse angle does not contact with the root canal wall.
Thus, the root canal wall does contact with the acute angle edge disposed on the shaft side between the two edges placed adjacent to each other, and the edge operates as a cutting edge having a large rake and a good grinding ability during the pulling manipulation, but during the pushing manipulation the edge comes to contact to the wall with a very small rake. Therefore, when the therapeutic instrument is pushed, a portion of the acute edge with a small rake contacts with the root canal wall, and this contact may rub the root canal wall but cannot grind the wall. When the therapeutic instrument is pulled, the root canal wall can be ground in a good state where a portion of the good grinding ability having a large rake comes in contact with the wall.
Therefore, the pushing manipulation of the therapeutic instrument does not produce shavings on the root apex side, so that no shaving is pressed in the direction of the root apex opening.
The above dental root canal therapeutic instrument (hereinafter, referred to as, simply, xe2x80x9ctherapeutic instrumentxe2x80x9d) may have a ratio of the longer side and shorter side of 1.5 or larger at the transversal cross section of the work portion. With such a structure, the values of the cross-sectional secondary moments are greatly different between in the direction along the longer side and in the direction along the shorter side. Therefore, an arbitrary portion of the work portion may have directionality in bending easiness. Because the work portion is formed in a spirally twisted manner, the work portion as a whole is not subject to the directionality in bending easiness, thereby being capable of providing high flexibility and rigidity.
Moreover, to solve the above problem, a dental root canal therapeutic instrument of the invention has a shaft portion and a work portion made of spiral projections in continuation with the shaft portion; the cross section of the work portion is formed of a rectangular constituted of a set of first edges having keen tips and a set of second edges having rounded tips, located as opposite angle, respectively; the first edges between the first and second edges, which form the projections, are placed on a side of the shaft on the side face of the work portion; the tip of the first edge is located at a further position with respect to the center axis of the work portion than the position of the tip of the second edge.
Where the cross section of the work portion is set to such a rectangular, only the first edge located on the shaft side may operate as a cutting edge by rendering the distance from the central axis to the second edge shorter than the distance to the first edge by rounding the tip of the second edge.